What are Refractories?

These materials are capable of withstanding high temperature in different industrial processes. In addition to the thermal endurance, a refractory should be able to resist the chemical action of the material being heated (molten metal slag in the case of metallurgical furnaces and the gases generated in the furnace) and withstand the mechanical load. They have high dimensional and chemical stability and do not lose their physical shape and chemical composition. Refractories confine the heat and prevent the heat loss to the atmosphere from the outside walls of furnaces. Refractories are not pure compounds and hence do not possess a sharp fusion point. Therefore, softening temperature is determined rather than fusion point. The ability of a material to withstand prolonged action of high temperature (1580° C and onwards) without appreciable softening under service condition is known as refractoriness expressed in degrees C. It is generally measured by the softening or the melting point of the material and is determined by the pyromeric cone method, with the aid of tetrahedral cones or elongated pyramids, made of the same material to be tested, the size being 20 mm base and 40 mm height. They are heated until they soften and bend to touch the base. These cones are then compared with the standard Segar cone of known refractoriness, kept under same thermal load. The value is known as Pyromeric Cone Equivalent (PCE). Fire-clay and high alumina clay soften gradually over a range of temperature, whereas, others silica softens over a relatively narrow range.